1. Field of the Invention
The present invention relates to a technique for accurately detecting a grammage of a recording medium used in an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as copiers or laser printers include a sensor for determining a type of a recording medium. A method which uses the sensor in determining a type of the recording medium and sets a transfer condition or a fixing condition according to a determination result has been discussed.
A method for detecting a thickness of a recording medium by detecting an amount of light transmitted through the recording medium has been discussed. Further, Japanese Patent Application Laid-Open No. 57-132055 discusses a method for detecting a grammage of a recording medium by emitting an ultrasonic wave. The method that uses the ultrasonic wave needs to consider a reflected ultrasonic wave which is emitted from an ultrasonic wave transmitting unit (hereinafter referred to as a transmitting unit) and reflected from a recording medium. Additionally, it is necessary to consider an influence of a reflected ultrasonic wave that is transmitted through the recording medium and reflected from an ultrasonic wave receiving unit (hereinafter referred to as a receiving unit) Further, it is necessary to consider an influence of an ultrasonic wave reflected from a member in the periphery of the transmitting unit or the receiving unit. The member is, for example, a conveyance roller or a conveyance guide for conveying the recording medium.
As a method for reducing the influence of such reflected waves, Japanese Patent Application Laid-Open No. 2001-351141 discusses a configuration in which a guide is arranged for each ultrasonic wave transmitting unit and ultrasonic wave receiving unit.
However, according to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-351141 in which a guide is arranged for each ultrasonic wave transmitting unit and ultrasonic wave receiving unit, interference may occur between an ultrasonic wave that is emitted from the ultrasonic wave transmitting unit and a reflected ultrasonic wave that is reflected by the guide before the ultrasonic wave reaches the recording medium. Due to such interference, the ultrasonic wave which is output from the transmitting unit may be emitted to the recording medium in an attenuated or an unstable state.
Further, interference also may occur between the ultrasonic wave that transmitted through the recording medium and a reflected ultrasonic wave that is reflected by the guide of the receiving unit before the ultrasonic wave reaches the receiving unit. Due to such interference, the ultrasonic wave is emitted to the recording medium in the attenuated or the unstable state. If the ultrasonic wave is attenuated or unstable, the grammage detection accuracy is decreased.